The present invention relates to a DC/DC converter, in particular, to a converter with an adjustable output that utilizes synchronous pulse-width modulated switching with over-current, over-voltage, and soft-start functions.
As portable electronic devices become more popular, there is a strong need for more efficient uses of limited energy sources such as batteries. This requirement has led to electronic devices to operate with lower voltage levels. For example many modern microprocessors only require between 1.3 v-3.5 v to operate. As many of circuits have a higher inputted voltage level, a DC-DC converter that alters a first input voltage to a second lower output voltage is required. DC-DC converters typically fall into two categories; linear regulators, and switching regulators. Of the two, switching regulators are the preferred devices for applications where energy sources are limited, due to their operating efficiency wherein efficiency is output energy over input energy. Switching regulators operate by passing energy in discrete packets over a low resistance switch. Types of switching regulators are usually defined by the methods in which their switches are controlled. The main methods include pulse-frequency modulation PFM where the switch is cycled at a 50% duty cycle until the output voltage comes within range, current-limited pulse-frequency modulation where the charge terminates when a predetermined peak inductor current is reached, and pulse-width modulation PWM where the switch frequency is constant and controlled through the duty cycle that varies with the load. In order to obtain maximum efficiency within the circuit, one of the largest power loss factors is the voltage drop across the diode of a switching regulator. To minimize this loss many designs utilize a Schottky-type diodes that have a low forward voltage drop and high speed. However for maximum efficiency MOSFETs can be used in a synchronous rectification design. The low on-resistance of the MOSFETs allows this design to have a minimum voltage drop so that conversion efficiency can be further improved. An example of a synchronous rectification circuit is shown in FIG. 1. A PWM control circuit controls the operation of the transistors as so neither transistor is on at the same time. The upper transistor is connected to a voltage supply that is impressed across the inductor when the upper transistor is on, and the lower transistor is off. Current through the inductor rises linearly with time at a rate that is proportional to the input voltage divided by the inductance. When the upper transistor turns off, the lower transistor turns on, and the voltage across the inductor changes instantaneously to whatever is required to maintain the current flow. A capacitor coupled to the output voltage helps to reduce the voltage ripple from the output.
Typically microprocessors will use linear power supplies which can provide the most noise free, stable supply, however its low conversion efficiency is not suitable for microprocessors in portable applications wherein the power consumption can require in excess of 30 W. Hence the use of switching power supplies for applications with limited power sources is an attractive alternative due to their energy efficiencies. Many switching powers supplies however are not suitable for modern day microprocessors due to their the high ripple in their outputs, poor dynamic response, and noise on the outputs that make them incompatible with manufacturer requirements for power sources for microprocessors. Therefore there is a need for a switching power supply that can adequately meet the power requirements of modem microprocessors.
It is therefore an object of the present invention to provide a synchronous PWM DC/DC converter circuit where the circuit integrates control, output adjustments, output monitoring and protection functions.
It is also an object of the present invention to provide a DC/DC converter that has high efficiency, high precision, fast transient response, a small start-up overshoot, over-current protection, and over-voltage protection so as to meet the requirements of a power source for modem microprocessors.
The invention has 5 digital inputs that can select up to 32 discrete output voltage levels. The circuit comprises a 5-bit DAC to covert the digital signal into an analog voltage level. The analog voltage is compared to the output voltage using an error amplifier. The output error level is inputted to a pulse-width modulator with a triangle waveform from a triangle waveform oscillator, which then outputs a PWM signal to a logical gate controller circuit. If the circuit is operating normally the PWM signal then drives the upper and lower output drivers. The drivers create non-overlapping synchronous that drive two power MOSFETs which act as switches to convert an inputted 5v level to a desired voltage level. The circuit provides a signal indicating if the output voltage is within +/xe2x88x9210% of an expected input voltage. The circuit includes over-current, and over-voltage protection as well as a soft-start circuit to control the rising of the output voltage.
These and other features, which characterize the invention, are set forth in the claims annexed hereto and forming a further part hereof. However, for a better understanding of the invention, and of the advantages and objectives attained through its use, reference should be made to the drawings, and to the accompanying descriptive matter, in which there is described exemplary embodiments of the invention.